Atolls are circular, oval, or horseshoe shaped coral reefs. They are islands that are formed around a volcanic seamount. They have a shallow central lagoon. They also make small islands that are separated from each other by channels of water that lead from the ocean into the lagoon. Atolls are usually low lying with an elevation of 5 meters or less. Atolls are formed when living corals gather and build a reef on the sides of a seamount or volcano or very small island. Over time the volcano is eroded away. The corals, whose algae require light to grow, continue to build the reef up towards the ocean surface. The reef gradually becomes separated from the sinking island by a lagoon, forming a barrier reef. Eventually the cold volcano sinks so far that it disappears beneath the surface of the ocean.
December of 1945, President Harry S. Truman issued a statement that joint testing of nuclear weapons would be necessary "to determine the effect of atomic bombs on American warships." Bikini atoll in the Marshall Islands, because of its location away from regular air and sea routes, was chosen to be the new nuclear proving ground for the United States government. On May 7 1946 the government bombed Bikini Atoll of the marshall islands.